Microwave reflector antennas have long been used as the primary means for transmitting high frequency communication signals to distant receivers. Most reflectors are parabolic, with a single focal point. Incoming plane waves that fall within the aperture of the antenna reflect off the conducting metal surfaces and are directed to this focal point. Consistent with the principle of reciprocity, waves originating from a feed (transmitter) located at the focal point will reflect off the metal surfaces to form an outgoing plane wave without phase error.
Incoming beams that arrive at a non-zero angle with respect to the bore-sight direction and are subsequently reflected by the antenna surface to a detector (receiver) at a focal point are said to be scanned. (The bore-sight direction is the axis of symmetry of the reflecting surface.) Conversely, when a feed is displaced from the focal point, the outgoing transmitted beam is angularly displaced (scanned) from the bore-sight direction. In this case, the field of an outgoing beam at the reflector aperture contains non-planar phase errors. These errors result in a degraded outgoing beam with reduced peak gain, increased sidelobe levels, and filled nulls.
The antenna's effective field of view is defined as the greatest angle at which beams can be scanned without being excessively degraded. Parabolic reflectors are limited to only a few beamwidths of scanning. With a typical focal length to aperture diameter ratio (F/D) of 0.5, these reflectors yield a peak gain scan loss of at least 10 dB at 20 half-power beamwidths corresponding to a field of view of about .+-.5.degree. for medium quality beams.
Attempts have been made to improve single reflector scanning capability by considering deformed geometries based on the sphere or parabolic torus. Unfortunately, although scanning capability does improve for these more circular geometries, the aperture efficiency (the ratio of usable reflector area to the area of the entire reflector aperture) becomes very low. To maintain acceptable beam quality, typically only a small portion of the much larger reflector area is illuminated by any single beam. Most of the reflector is unused unless close multiple beams are employed.
Dragone, U.S. Pat. No. 4,786,910, discloses an antenna including an ellipsoidal reflecting surface with two focal points and multiple feeds disposed so as to yield scanned beams with a minimum acceptable level of astigmatic aberration. The surface of Dragone is an ellipsoid, i.e., any cross section has the shape of an ellipse. As a consequence, the surface can never be configured to minimize phase error aberrations for all scanned beams within a field of view of .+-.30.degree.. Also, this design exhibits high astigmatism for unscanned (on-axis) beams.